Loving this thread. It's keeping me occupied till the next episode.
I suppose Emily could have had something to do with the disappearance as we don't know what she was doing when Ollie went missing. I assumed that the whole family went back to the hotel after they'd finished their meal. Tony and Ollie packed up their swimming things and Emily waited at the hotel until they (or rather just Tony) came back.
But. I can't imagine any mother organising for her child to disappear when it would mean that she didn't see them for years. Emily can't really be seeing Ollie secretly, she is engaged to a policeman.
Also, she is shown to be genuinely upset. I have taken her rather frozen expressions to be her trying to suppress emotion and "move on". Though botox is probably more likely.
And I can't understand what motive she might have.
It's hard to engineer a car breakdown I would think. You would have to have had access to the car at the last stop (ferry, hotel, café?) and then tinker with it to make sure that it started properly but then wouldn't restart.
They stopped near Hotel Eden because Tony took a work call and the car wouldn't restart. So the place they stopped was fairly random. So, either someone had planned to kidnap Ollie and was following the family looking for an opportunity. Or, a person or people connected to the town was looking for a likely child to take and Ollie was in the wrong place at the wrong time. Or, another child (Mark's?) was meant to be taken and Ollie was taken by mistake.
Could Mark's wife have tried to organise snatching her child and Ollie was taken by mistake? He looks like Ollie, is the same age, also with a British father, staying in the same town? From the back even Emily thought he was Ollie.